Methods of forming semiconductor mesa structures including self-aligned contact layers and related devices

ABSTRACT

A method of forming a semiconductor device may include forming a semiconductor layer on a substrate, and forming a contact layer on the semiconductor layer opposite the substrate. After forming the semiconductor layer and the contact layer, the contact layer and the semiconductor layer may be patterned such that the semiconductor layer includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate and so that the patterned contact layer is on the mesa surface. Related structures and devices are also discussed.

RELATED APPLICATIONS

[0001] The present application claims the benefit of; U.S. Provisional Application No. 60/435,213 filed Dec. 20, 2002, and entitled “Laser Diode With Self-Aligned Index Guide And Via”; U.S. Provisional Application No. 60/434,914 filed Dec. 20, 2002, and entitled “Laser Diode With Surface Depressed Ridge Waveguide”; U.S. Provisional Application No. 60/434,999 filed Dec. 20, 2002 and entitled “Laser Diode with Etched Mesa Structure”; and U.S. Provisional Application No. 60/435,211 filed Dec. 20, 2002, and entitled “Laser Diode With Metal Current Spreading Layer.” The disclosures of each of these provisional applications are hereby incorporated herein in their entirety by reference.

[0002] The present application is also related to: U.S. application Ser. No. ______ (Attorney Docket No. 5308-281) entitled “Methods Of Forming Semiconductor Devices Having Self Aligned Semiconductor Mesas and Contact Layers And Related Devices” filed concurrently herewith; U.S. Application No. ______ (Attorney Docket No. 5308-282) entitled “Methods Of Forming Semiconductor Devices Including Mesa Structures And Multiple Passivation Layers And Related Devices” filed concurrently herewith; and U.S. application Ser. No. ______ (Attorney Docket No. 5308-283) entitled “Methods OfForming Electronic Devices Including Semiconductor Mesa Structures And Conductivity Junctions And Related Devices” filed concurrently herewith. The disclosures of each of these U.S. Applications are hereby incorporated herein in their entirety by reference.

FIELD OF THE INVENTION

[0003] The present invention relates to the field of electronics, and more particularly, to methods of forming electronic semiconductor devices and related structures.

BACKGROUND OF THE INVENTION

[0004] A laser is a device that produces a beam of coherent monochromatic light as a result of stimulated emission of photons. Stimulated emission of photons may also produce optical gain, which may cause light beams produced by lasers to have a high optical energy. A number of materials are capable of producing the lasing effect and include certain high-purity crystals (ruby is a common example), semiconductors, certain types of glass, certain gases including carbon dioxide, helium, argon and neon, and certain plasmas.

[0005] More recently, lasers have been developed in semiconducting materials, thus taking advantage of the smaller size, lower cost and other related advantages typically associated with semiconductor devices. In the semiconductor arts, devices in which photons play a major role are referred to as “photonic” or “optoelectronic” devices. In turn, photonic devices include light-emitting diodes (LEDs), photodetectors, photovoltaic devices, and semiconductor lasers.

[0006] Semiconductor lasers are similar to other lasers in that the emitted radiation has spatial and temporal coherence. As noted above, laser radiation is highly monochromatic (i.e., of narrow band width) and it produces highly directional beams of light. Semiconductor lasers may differ, however, from other lasers in several respects. For example, in semiconductor lasers, the quantum transitions are associated with the band properties of materials; semiconductor lasers may be very compact in size, may have very narrow active regions and larger divergence of the laser beam; the characteristics of a semiconductor laser may be strongly influenced by the properties of the junction medium; and for P-N junction lasers, the lasing action is produced by passing a forward current through the diode itself. Overall, semiconductor lasers can provide very efficient systems that may be controlled by modulating the current directed across the devices. Additionally, because semiconductor lasers can have very short photon lifetimes, they may be used to produce high-frequency modulation. In turn, the compact size and capability for such high-frequency modulation may make semiconductor lasers an important light source for optical fiber communications.

[0007] In broad terms, the structure of a semiconductor laser should provide optical confinement to create a resonant cavity in which light amplification may occur, and electrical confinement to produce high current densities to cause stimulated emission to occur. Additionally, to produce the laser effect (stimulated emission of radiation), the semiconductor may be a direct bandgap material rather than an indirect bandgap material. As known to those familiar with semiconductor characteristics, a direct bandgap material is one in which an electron's transition from the valence band to the conduction band does not require a change in crystal momentum for the electron. Gallium arsenide and gallium nitride are examples of direct bandgap semiconductors. In indirect bandgap semiconductors, the alternative situation exists; i.e., a change of crystal momentum is required for an electron's transition between the valence and conduction bands. Silicon and silicon carbide are examples of such indirect semiconductors.

[0008] A useful explanation of the theory, structure and operation of semiconductor lasers, including optical and electronic confinement and mirroring, is given by Sze, Physics of Semiconductor Devices, 2nd Edition (1981) at pages 704-742, and these pages are incorporated entirely herein by reference.

[0009] As known to those familiar with photonic devices such as LEDs and lasers, the frequency of electromagnetic radiation (i.e., the photons) that can be produced by a given semiconductor material may be a function of the material's bandgap. Smaller bandgaps produce lower energy, longer wavelength photons, while wider bandgap materials produce higher energy, shorter wavelength photons. For example, one semiconductor commonly used for lasers is aluminum indium gallium phosphide (AlInGaP). Because of this material's bandgap (actually a range of bandgaps depending upon the mole or atomic fraction of each element present), the light that AlInGaP can produce may be limited to the red portion of the visible spectrum, i.e., about 600 to 700 nanometers (nm). In order to produce photons that have wavelengths in the blue or ultraviolet portions of the spectrum, semiconductor materials having relatively large bandgaps may be used. Group III-nitride materials such as gallium nitride (GaN), the ternary alloys indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN) and aluminum indium nitride (AlInN) as well as the quaternary alloy aluminum gallium indium nitride (AlInGaN) are attractive candidate materials for blue and UV lasers because of their relatively high bandgap (3.36 eV at room temperature for GaN). Accordingly, Group III-nitride based laser diodes have been demonstrated that emit light in the 360-460 nm range.

[0010] A number of commonly assigned patents and co-pending patent applications likewise discuss the design and manufacture of optoelectronic devices. For example, U.S. Pat. Nos. 6,459,100; 6,373,077; 6,201,262; 6,187,606; 5,912,477; and 5,416,342 describe various methods and structures for gallium-nitride based optoelectronic devices. U.S. Pat. No. 5,838,706 describes low-strain nitride laser diode structures. Published U.S. Application Nos. 20020093020 and 20020022290 describe epitaxial structures for nitride-based optoelectronic devices. Various metal contact structures and bonding methods, including flip-chip bonding methods, are described in Published U.S. Application No. 20020123164 as well as Published U.S. Application No. 030045015 entitled “Flip Chip Bonding of Light Emitting Devices and Light Emitting Devices Suitable for Flip-Chip Bonding”; Published U.S. Application No. 20030042507 entitled “Bonding of Light Emitting Diodes Having Shaped Substrates and Collets for Bonding of Light Emitting Diodes Having Shaped Substrates”, and Published U.S. Application No. 20030015721 entitled “Light Emitting Diodes Including Modifications for Submount Bonding and Manufacturing Methods Therefor.” Dry etching methods are described in U.S. Pat. No. 6,475,889. Passivation methods for nitride optoelectronic devices are described in U.S. application Ser. No. 08/920,409 entitled “Robust Group III Light Emitting Diode for High Reliability in Standard Packaging Applications” and Published U.S. Application No. 20030025121 entitled “Robust Group III Light Emitting Diode for High Reliability in Standard Packaging Applications.” Active layer structures suitable for use in nitride laser diodes are described in Published U.S. Application No. 20030006418 entitled “Group III Nitride Based Light Emitting Diode Structures with a Quantum Well and Superlattice, Group III Nitride Based Quantum Well Structures and Group III Nitride Based Superlattice Structures” and Published U.S. Application No. 20030020061 entitled “Ultraviolet Light Emitting Diode.” The contents of all of the foregoing patents, patent applications and published patent applications are incorporated entirely herein by reference as if fully set forth herein.

[0011] Moreover, laser diodes may require relatively high current levels to provide conditions for lasing. Accordingly, non-uniformities in distributions of current across an active region of a laser diode may reduce performance thereof.

SUMMARY

[0012] According to embodiments of the present invention, methods of forming semiconductor devices may include forming a semiconductor layer on a substrate, and forming a contact layer on the semiconductor layer opposite the substrate. After forming the semiconductor layer and the contact layer, the contact layer and the semiconductor layer can be patterned such that the semiconductor layer includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate and so that the patterned contact layer is on the mesa surface.

[0013] More particularly, the mesa may be configured to provide at least one of optical confinement or current confinement for a light emitting device in the patterned semiconductor layer. Moreover, the mesa sidewalls may be free of the contact layer.

[0014] Patterning the contact layer and the semiconductor layer may include forming a mask layer on the contact layer opposite the semiconductor layer, and etching portions of the contact layer and the semiconductor layer exposed by the mask layer. After patterning the contact layer and the semiconductor layer, a passivation layer can be formed on the mesa sidewalls and on the mesa surface so that the passivation layer is on at least a portion of the patterned contact layer opposite the patterned semiconductor layer. Moreover, forming the passivation layer may include forming the passivation layer across the contact layer opposite the substrate, and a via may be formed in the passivation layer exposing a portion of the contact layer opposite the mesa surface. In addition, a metal layer may be formed on the passivation layer and on the exposed portions of the contact layer opposite the mesa surface.

[0015] The contact layer may substantially cover an entirety of the mesa surface, and the semiconductor layer may include a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa. The semiconductor layer may also include an active layer between the P-type layer and the N-type layer, and a second contact layer electrically coupled with the mesa may be formed so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer. In addition, the N-type layer may be between the P-type layer and the substrate, and the P-type layer may be between the N-type layer and the contact layer.

[0016] The contact layer may be a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium, and the semiconductor layer may include an epitaxial semiconductor material. The semiconductor layer may include a Group III-V semiconductor material, and the Group III-V semiconductor material may be a Group III-nitride semiconductor material.

[0017] According to additional embodiments of the present invention, methods for forming semiconductor devices may include forming a semiconductor structure on a substrate wherein the semiconductor structure includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate. A contact layer may be formed on the mesa surface, and a passivation layer may be formed on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface. Moreover, the passivation layer may have a via hole therein exposing a portion of the contact layer opposite the mesa surface.

[0018] The mesa may be configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure. Moreover, the mesa sidewalls may be free of the contact layer.

[0019] The contact layer may substantially cover an entirety of the mesa surface, and the semiconductor structure may include a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa. The semiconductor structure may also include an active layer between the P-type layer and the N-type layer. A second contact layer electrically coupled with the semiconductor structure may be formed so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer. In addition, the N-type layer may be between the P-type layer and the substrate, and the P-type layer may be between the N-type layer and the contact layer.

[0020] A metal layer may also be formed on the passivation layer and on the exposed portions of the contact layer opposite the semiconductor layer, and the contact layer may comprise a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium. Moreover, the semiconductor layer may include an epitaxial semiconductor material such as a Group III-V semiconductor material, and more particularly, a Group III-nitride semiconductor material.

[0021] According to still additional embodiments of the present invention, methods of forming a semiconductor device may include forming a semiconductor structure on a substrate wherein the semiconductor structure includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate. In addition, a contact layer may be formed substantially covering an entirety of the mesa surface opposite the substrate. A passivation layer may also be formed on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface wherein the passivation layer has a via hole therein so that a portion of the contact layer opposite the mesa surface is free of the passivation layer. A metal layer may also be formed on the passivation layer and on the portion of the contact layer free of the passivation layer,

[0022] More particularly, the mesa may be configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure. The mesa sidewalls may also be free of the contact layer.

[0023] In addition, the semiconductor structure may include a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa. The semiconductor structure may further include an active layer between the P-type layer and the N-type layer, and a second contact layer electrically coupled with the semiconductor structure may be formed so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer. Moreover, the N-type layer may be between the P-type layer and the substrate, and the P-type layer may be between the N-type layer and the contact layer.

[0024] The contact layer may include a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium, and the semiconductor layer may include an epitaxial semiconductor material such as a Group III-V semiconductor material, and more particularly, a Group III-nitride semiconductor material.

[0025] According to yet additional embodiments of the present invention, a semiconductor device may include a semiconductor structure on a substrate wherein the semiconductor structure includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate. A contact layer may be included on the mesa surface, and a passivation layer may be included on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface. In addition, the passivation layer may have a via hole therein exposing a portion of the contact layer opposite the mesa surface.

[0026] The mesa may also be configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure. In addition, the mesa sidewalls may be free of the contact layer.

[0027] The contact layer may substantially cover an entirety of the mesa surface, and the semiconductor structure may include a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa. The semiconductor structure may also include an active layer between the P-type layer and the N-type layer. In addition, a second contact layer may be electrically coupled with the semiconductor structure so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer. Furthermore, the N-type layer may be between the P-type layer and the substrate, and the P-type layer may be between the N-type layer and the contact layer.

[0028] In addition, a metal layer may be provided on the passivation layer and on the exposed portions of the contact layer opposite the semiconductor layer, and the contact layer may include a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium. The semiconductor layer may include an epitaxial semiconductor material such as a Group III-V semiconductor material, and more particularly, a Group III-nitride semiconductor material.

[0029] According to more embodiments of the present invention, a semiconductor device may include a semiconductor structure on a substrate wherein the semiconductor structure includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate. A contact layer may substantially cover an entirety of the mesa surface opposite the substrate.

[0030] The mesa may also be configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure. Moreover, the mesa sidewalls may be free of the contact layer.

[0031] In addition, a passivation layer may be provided on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface wherein the passivation layer has a via hole therein exposing a portion of the contact layer opposite the mesa surface. A metal layer may also be provided on the passivation layer and on the exposed portion of the contact layer opposite the semiconductor layer.

[0032] The semiconductor structure may include a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa. The semiconductor structure may further include an active layer between the P-type layer and the N-type layer. In addition, a second contact layer may be electrically coupled with the semiconductor structure so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer. More particularly, the N-type layer may be between the P-type layer and the substrate, and the P-type layer may be between the N-type layer and the contact layer.

[0033] The contact layer may include a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium, and the semiconductor layer may include an epitaxial semiconductor material such as a Group III-V semiconductor material, and more particularly, a Group III-nitride semiconductor material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a cross-sectional view illustrating a mesa structure for a semiconductor laser.

[0035]FIG. 2 is a cross-sectional view illustrating semiconductor structures according to embodiments of the present invention.

[0036]FIG. 3 is a scanning electron microscope (SEM) photomicrograph of a cross-section of a semiconductor structure according to embodiments of the present invention.

[0037] FIGS. 4A-E are cross-sectional views illustrating steps of forming semiconductor structures according to embodiments of the present invention.

DETAILED DESCRIPTION

[0038] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. It will also be understood that when an element is referred to as being “coupled” or “connected” to another element, it can be directly coupled or connected to the other element, or intervening elements may also be present. Like numbers refer to like elements throughout. Furthermore, relative terms such as “vertical” and “horizontal” may be used herein to describe a relationship with respect to a substrate or base layer as illustrated in the FIG.s. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

[0039] Group III-nitride materials may be made P-type by doping with P-type impurities (dopants) such as magnesium. However, P-type nitride semiconductor materials may provide relatively low carrier activation rates and relatively low carrier mobilities. Accordingly, P-type nitride semiconductor materials may be characterized by relatively high resistivities. Because laser diodes may require relatively high current levels to provide conditions for lasing, it may be beneficial for the ohmic contact to the P-type nitride material to cover as much surface area as possible.

[0040]FIG. 1 is cross-sectional view illustrating a structure providing an ohmic contact to a P-type Group III-nitride based laser diode. As shown in FIG. 1, a laser structure 210 includes a substrate 212 on which is formed an epitaxial semiconductor structure 214 comprising one or more Group III-nitride materials. The epitaxial semiconductor structure 214 may include an N-type layer 215, a P-type layer 217, and an active layer 216 between the N-type and P-type layers. The active layer 216 may include any of a number of different structures and/or layers and/or combinations thereof, such as single or multiple quantum wells, double heterostructures, and/or superlattices. Active layer 216 may also include light and current confinement layers that may encourage laser action in the device.

[0041] Portions of the epitaxial structure 214 may be patterned into a mesa structure 220 for optical and current confinement purposes. A passivation layer 218 may protect and insulate exposed surfaces of the P-type layer 217. The passivation layer 218 may be a layer of an insulating material such as silicon dioxide, silicon nitride, aluminum oxide, and/or combinations thereof.

[0042] The laser structure 210 may include a first ohmic contact layer 226 on the P-type layer 217 and a second ohmic contact layer 227 on the substrate 212 opposite the epitaxial semiconductor structure 214. A metal overlayer 224 may be provided on the passivation layer 218 and on the first ohmic contact layer 226 to provide a conductive path for interconnection of the device 210 with an external circuit.

[0043] While the second ohmic contact 227 is shown on the substrate 212, the ohmic contact 227 may be provided on the N-type layer 215. In the device illustrated in FIG. 1, the substrate 212 may comprise a conductive material such as N-type silicon carbide to provide a “vertical” device having a “vertical” current path between the first and second ohmic contacts 226 and 227 through the epitaxial semiconductor structure 214 and the substrate 212. Stated in other words, the anode and cathode of the device are on opposite sides of the substrate 212. In a “horizontal” device, for example, the second ohmic contact could be placed on an exposed portion of the N-type layer 215 so that both ohmic contacts are on the same side of the substrate.

[0044] As shown in FIG. 1, the ohmic contact 226 on the P-type layer 217 can be formed within a via 222 that has been opened through the passivation layer 218 to expose a portion of the surface 220A of the mesa 220. More particularly, the mesa 220 can be fabricated by forming an epitaxial semiconductor layer, forming a photoresist layer on the epitaxial semiconductor layer, patterning the photoresist layer to expose portions of the semiconductor layer (using a technique known as photolithography), and etching the exposed portions of the epitaxial semiconductor layer to form the mesa 220. The epitaxial semiconductor layer can be etched using a dry etch in an argon (Ar) environment using an etchant including chlorine (Cl₂). More particularly, a dry etch for the epitaxial semiconductor layer may include flowing argon (Ar) at a rate in the range of approximately 2-40 sccm and flowing chlorine (Cl₂) at a rate in the range of approximately 5-50 sccm in a reactive ion etch (RIE) reactor at a pressure in the range of approximately 5-50 mTorr and at a radio frequency (RF) power in the range of approximately 25-1000 W.

[0045] The epitaxial semiconductor structure 214 including the mesa 220 is then covered with the passivation layer 218, and a second patterned photoresist layer can be formed and patterned (using photolithography) on the passivation layer to expose a portion of the passivation layer where the via is to be formed. The exposed portion of the passivation layer can then be etched to form the via 222 exposing a portion of the mesa surface 220A.

[0046] A layer of a metal such as nickel, titanium, platinum, palladium, and/or combinations thereof can then be deposited on the portion of the mesa surface 220A exposed by the via 222. Because of tolerance limits of the two photolithography steps discussed above, however, it may be difficult to align the via 222 with the mesa surface 220A. Accordingly, the via 222 may need to be patterned to be significantly narrower than the mesa surface 220A so that the passivation layer 218 may extend onto significant portions of the mesa surface 220A and so that the ohmic contact 226 may not contact significant portions of the mesa surface 220A. Accordingly, electric current passing from the ohmic contact 226 to the mesa surface 220A may be distributed unevenly across the mesa and performance of the device may be degraded.

[0047] As shown in FIG. 1, the passivation layer 218 may cover corners 211 of the mesa 220. The corners 211 may be electrically vulnerable regions of the structure, and the passivation layer may provide protection therefor. More particularly, it may be desirable to protect the mesa corners 211 when the metal overlayer 224 is deposited. If the corners are not protected when the metal overlayer 224 is deposited, metal from the overlayer may migrate down the mesa 220 sidewalls which may cause current leakage, electrical short circuits, and/or an increase in a lasing threshold voltage and/or current. Providing portions 228 of the passivation layer 218 on the corners 211 of the mesa 220A may also protect mesa sidewalls from environmental conditions such as high humidity.

[0048] A laser diode structure according to embodiments of the present invention is illustrated in the cross-sectional view of FIG. 2. The laser diode structure may include a substrate 12, an epitaxial semiconductor structure 14, ohmic contact layers 36 and 27, a passivation layer 34, and a metal overlayer 24. Moreover, the epitaxial semiconductor structure 14 may include a Group III-V compound semiconductor material such as a Group III-nitride compound semiconductor material. The ohmic contact layers 36 and 27 may each comprises a layer of a metal such as aluminum, copper, gold, nickel (Ni), titanium (Ti), platinum (Pt), and/or palladium (Pd). The metal overlayer 24 may comprise a layer of a metal such as nickel (Ni), gold (Au), platinum (Pt), titanium (Ti), tungsten (W), molybdenum (Mo), tantalum (Ta), and/or palladium (Pd).

[0049] In some embodiments, the substrate 12 may include substrate materials such as N-type silicon carbide having a polytype such as 2H, 4H, 6H, 8H, 15R, and/or 3C; sapphire; gallium nitride; and/or aluminum nitride. Moreover, the substrate 12 may be conductive to provide a “vertical” device having a “vertical” current flow through the epitaxial semiconductor structure 14 and the substrate 12. In an alternative, the substrate 12 may be insulating or semi-insulating where both ohmic contacts are provided on a same side of the substrate to provide a “horizontal” device. A conductive substrate could also be used in a “horizontal” device. Moreover, the term substrate may be defined to include a non-patterned portion of the semiconductor material making up the semiconductor structure 14, and/or there may not be a material transition between the substrate 12 and the semiconductor structure 14.

[0050] Portions of the epitaxial semiconductor structure 14 may be patterned into a mesa stripe, for example, to provide optical and/or current confinement. As shown, only a portion of the epitaxial semiconductor structure 14 is included in the mesa 20. For example, the epitaxial semiconductor structure 14 may include N-type and P-type layers and portions of one or both of the N-type and P-type layers may be included in the mesa 20. According to particular embodiments, the epitaxial semiconductor structure 14 may include an N-type layer 15 adjacent the substrate 12 and a P-type layer 17 on the N-type layer opposite the substrate 12. The mesa may include portions of the P-type layer 17 and none of the N-type layer 15 as shown in FIG. 2. In alternatives, the mesa may include all of the P-type layer 17 and portions (but not all) of the N-type layer; or all of the P-type layer 17 and the N-type layer 15 (such that sidewalls of the mesa 20 extend to the substrate 12.

[0051] The epitaxial semiconductor structure 14 may also include an active layer 16 between the N-type layer 15 and the P-type layer 17. The active layer 16 may include a number of different structures and/or layers and/or combinations thereof. The active layer 16, for example, may include single or multiple quantum wells, double heterostructures, and/or superlattices. The active layer 16 may also include light and/or current confinement layers that may encourage laser action in the device.

[0052] By way of example, a uniformly thick layer of epitaxial semiconductor material may be formed on the substrate 12, and a layer of an ohmic contact material may be formed on the layer of the epitaxial semiconductor material. The mesa 20 and the ohmic contact layer 36 may be formed by selectively etching the layer of the contact material and the layer of the epitaxial semiconductor material using the same etch mask. Moreover, a height of the mesa 20 may be determined by a depth of the etch used to form the mesa 20. According to embodiments of the present invention, the mesa etch depth (and resulting mesa thickness) may be in the range of approximately 0.1 to 5 microns, and according to additional embodiments may be no greater than approximately 2.5 microns. In addition, a width of the mesa surface 20A between mesa sidewalls 20B may be in the range of approximately 1 to 10 microns or more. By patterning the ohmic contact layer 36 and the mesa 20 using the same etch mask, the ohmic contact layer 36 may substantially cover an entirety of the mesa surface 20A between mesa sidewalls 20B. Moreover, the mesa surface 20A may be a P-type semiconductor material.

[0053] The passivation layer 34 may protect and insulate the epitaxial semiconductor structure 14 including the mesa 20. The passivation layer 34, for example, may include a layer of an insulating material such as silicon dioxide, silicon nitride, aluminum oxide, and/or combinations thereof, and the passivation layer 34 may be formed using a deposition technique such as plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD), chemical vapor deposition (CVD), sputtering, and/or e-beam evaporation. A via 32 through the passivation layer 34 may expose a portion of the ohmic contact layer 36, and the metal overlayer 24 may contact the ohmic contact layer 36 through the via 32. As shown, portions 38 of the passivation layer 34 may overlap peripheral portions of the ohmic contact layer 36 opposite the mesa surface 20A, and portions of the ohmic contact layer 36 exposed by the via 32 may be free of the passivation layer 34.

[0054]FIG. 3 is a scanning electron microscope (SEM) micrograph of a semiconductor structure according to embodiments of the present invention. As shown, the semiconductor structure includes a substrate 12′, an epitaxial semiconductor structure 141 including a mesa 20′ having a mesa surface 20A′, an ohmic contact layer 36′, a passivation layer 34′, and a metal overlayer 24′. As shown in FIG. 3, the epitaxial semiconductor structure 14′ may include an N-type layer 15′ and a P-type layer 17′, and sidewalls 20B′ of the mesa 20′ may extend to the substrate 12′ so that all of the epitaxial semiconductor structure 14′ is included in the mesa 20′.

[0055] The passivation layer 34′ may be a layer of silicon nitride that may provide protection and insulation for exposed surfaces of the epitaxial semiconductor structure 14′ including mesa 20′. Via 32′ through the passivation layer 34′ may expose a portion of the ohmic contact layer 36′ so that the exposed portion of the ohmic contact layer 36′ is free of the passivation layer 34′. The metal overlayer 24′ contacts the ohmic contact layer 36′ through the via 32′. Portions 38′ of the passivation layer 34′ overlapping peripheral portions of the ohmic contact layer 36′ may provide protection for the peripheral portions of the ohmic contact layer 36′ and corner portions of the mesa 20′ wherein the mesa surface 20A′ and the mesa sidewalls 20B′ meet.

[0056] Because the ohmic contact layer 36′ may cover substantially an entirety of the mesa surface 20A′ between mesa sidewalls 20B′, electric current passing between the metal overlayer 24′ and the mesa 20′ may be spread substantially over an entirety of the width of the mesa surface 20A′ between mesa sidewalls 20B′ using the ohmic contact layer 36′ without using a current spreading layer in the P-type layer 17′. In other words, the ohmic contact layer 36′ may act as a current spreading layer thereby improving current-carrying characteristics of the semiconductor device of FIG. 3 by spreading current outside the P-type layer 17′ of mesa 20′. By providing the ohmic contact layer 36′ as a current spreading layer, current flow through the epitaxial region may be improved thereby enhancing light emission from the laser diode.

[0057] Steps of forming semiconductor devices according to embodiments of the present invention are illustrated in FIGS. 4A-E. As shown in FIG. 4A, a precursor structure of a semiconductor device, such as a laser diode, may include a precursor epitaxial semiconductor layer 114′ on substrate 112 and a precursor ohmic contact layer 142′ on the precursor epitaxial semiconductor layer 114′. The precursor ohmic contact layer 142′ may include a metal stack that provides ohmic contact with the epitaxial semiconductor layer. In addition to or in an alternative to providing ohmic contact with the epitaxial semiconductor layer a metal stack of the precursor ohmic contact layer 142′ may also include other layers such as barrier and/or bonding layers as described, for example, in Published U.S. Patent Application No. 20030045015 (Ser.No. 10/185,252) and Published U.S. Patent Application No. 20030042507 (Ser.No. 10/185,350), the disclosures of which are hereby incorporated herein in their entirety by reference.

[0058] A mask 144 can be provided on the precursor ohmic contact layer 142′ so that portions of the precursor ohmic contact layer 142′ and the precursor epitaxial semiconductor layer 114′ are free of the mask layer. For example, the mask 144 may be a photoresist mask that is patterned using photolithographic techniques. In an alternative, the mask 144 may be a layer of another material that can resist an etch chemistry used to etch the precursor ohmic contact layer 142′ and the precursor epitaxial semiconductor layer 114′.

[0059] In addition, the precursor epitaxial semiconductor layer 114′ may include an N-type layer adjacent the substrate 112 and a P-type layer on the N-type layer opposite the substrate 112. The precursor epitaxial semiconductor layer 114′ may also include an active layer between N-type and P-type layers. An active layer, for example, may include a number of different structures and/or layers and/or combinations thereof. An active layer, for example, may include single or multiple quantum wells, double heterostructures, and/or superlattices. An active layer may also include light and/or current confinement layers that may encourage laser action in the completed device.

[0060] Portions of the precursor ohmic contact layer 142′ and the precursor epitaxial semiconductor layer 114′ not covered by the mask 144 may be selectively removed to provide ohmic contact layer 142 and epitaxial semiconductor layer 114. More particularly, the epitaxial semiconductor layer 114 may define a mesa 146 having a mesa surface 146A opposite the substrate and mesa sidewalls 146B between the mesa surface 146A and the substrate 112, and the ohmic contact layer 142 may extend across substantially an entire width of the mesa surface 146A between mesa sidewalls 146B.

[0061] Because the ohmic contact layer 142 and the epitaxial semiconductor layer 114 are patterned using the same mask 144, the ohmic contact layer 142 may be “self-aligned” with respect to the mesa surface 146A of the mesa 146. Accordingly, the ohmic contact layer 142 may extend across substantially an entire width of the mesa surface 146A between mesa sidewalls 146B without extending onto mesa sidewalls 146B. The ohmic contact layer 142 may thus spread current across substantially an entire width of the mesa surface 146A between mesa sidewalls 146B without shorting to mesa sidewalls-146B.

[0062] As shown in FIG. 4B, the etch depth may be such that the epitaxial semiconductor layer 114 may be etched to the substrate 112 so that mesa sidewalls 146B extend to the substrate 112. If the epitaxial semiconductor layer 114 includes N-type and P-type layers, an entirety of both the N-type and P-type layers may be included in the mesa when the mesa sidewalls extend to the substrate. In an alternative, the semiconductor layer 114 may not be etched completely to the substrate so that the mesa does not include all of the semiconductor layer. If the semiconductor layer includes N-type and P-type layers, a portion of one or both layers may be included in the mesa and a portion of one or both layers may be included in an unpatterned portion of the semiconductor layer adjacent the substrate.

[0063] As shown in FIG. 4C, the mask 144 can be removed, and a passivation layer 148 may be formed on the ohmic contact layer 142, on sidewalls of the mesa 146, and on the substrate 112. While the passivation layer 148 is shown directly on portions of the substrate 112, portions of the epitaxial semiconductor layer 114 may be between the passivation layer 148 and the substrate 112 if sidewalls of the mesa 146 do not extend to the substrate surface. The passivation layer 148 may be a layer of an insulating material such as silicon nitride, silicon dioxide, aluminum oxide, and/or combinations thereof, and the passivation layer 148 may be formed using a deposition technique such as plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD), chemical vapor deposition (CVD), sputtering, and/or e-beam evaporation. Moreover, the passivation layer 148 may be formed having a thickness in the range of approximately 0.1 to 2 microns.

[0064] A via 150 can then be formed in the passivation layer 148 using photolithographic patterning techniques to thereby expose a portion 142A of the ohmic contact layer 142. Stated in other words, exposed portions 142A of the ohmic contact layer 142 are free of the passivation layer 148 after forming the via 150. Because the ohmic contact layer 142 is patterned before forming the passivation layer 148, a tolerance for positioning of the via 150 does not affect a tolerance of an alignment of the ohmic contact layer 142 with respect to the mesa surface 120A. Moreover, portions of the passivation layer 148 extending on the ohmic contact layer 142 adjacent the via 150 may provide protection for corner portions of the mesa 146.

[0065] As shown in FIG. 4E, a metal overlayer 152 may be deposited on the passivation layer 148 and on portions of the ohmic contact layer 142 free of the passivation layer 148. The metal overlayer 150 may be a layer of a metal such as nickel, gold, platinum, titanium, molybdenum, tantalum, palladium, and/or combinations thereof. Accordingly, electrical connection to another device may be provided on the metal overlayer 152 at a point relatively remote from the mesa 146.

[0066] The resulting semiconductor device may provide an edge emitting semiconductor laser with light being emitted parallel to the substrate along a lengthwise direction of a semiconductor mesa stripe. Stated in other words, the light may be emitted along a direction perpendicular to the cross section of FIG. 4E. While methods and devices have been discussed with reference to methods of forming light emitting devices such as laser diodes, methods according to embodiments of the present invention may be used to form other semiconductor devices such as conventional diodes, conventional light emitting diodes, or any other semiconductor device including a semiconductor mesa.

[0067] While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents. 

That which is claimed is:
 1. A method of forming a semiconductor device, the method comprising: forming a semiconductor layer on a substrate; forming a contact layer on the semiconductor layer opposite the substrate; and after forming the semiconductor layer and the contact layer, patterning the contact layer and the semiconductor layer such that the semiconductor layer includes a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate and so that the patterned contact layer is on the mesa surface.
 2. A method according to claim 1 wherein patterning the contact layer and the semiconductor layer comprises, forming a mask layer on the contact layer opposite the semiconductor layer, and etching portions of the contact layer and the semiconductor layer exposed by the mask layer.
 3. A method according to claim 1 further comprising: after patterning the contact layer and the semiconductor layer, forming a passivation layer on the mesa sidewalls and on the mesa surface so that the passivation layer is on at least a portion of the patterned contact layer opposite the patterned semiconductor layer.
 4. A method according to claim 3 wherein forming the passivation layer comprises forming the passivation layer across the contact layer opposite the substrate, the method further comprising: forming a via in the passivation layer exposing a portion of the contact layer opposite the mesa surface.
 5. A method according to claim 3 further comprising: forming a metal layer on the passivation layer and on the exposed portions of the contact layer opposite the mesa surface.
 6. A method according to claim 1 wherein the contact layer substantially covers an entirety of the mesa surface.
 7. A method according to claim 1 wherein the semiconductor layer includes a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa.
 8. A method according to claim 7 wherein the semiconductor layer further includes an active layer between the P-type layer and the N-type layer.
 9. A method according to claim 7 further comprising: forming a second contact layer electrically coupled with the mesa so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer.
 10. A method according to claim 7 wherein the N-type layer is between the P-type layer and the substrate and wherein the P-type layer is between the N-type layer and the contact layer.
 11. A method according to claim 1 wherein the contact layer comprises a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium.
 12. A method according to claim 1 wherein the semiconductor layer comprises an epitaxial semiconductor layer.
 13. A method according to claim 1 wherein the semiconductor layer comprises a Group III-V semiconductor material.
 14. A method according to claim 13 wherein the Group III-V semiconductor material comprises a Group III-nitride semiconductor material.
 15. A method according to claim 1 wherein the mesa is configured to provide at least one of optical confinement or current confinement for a light emitting device in the patterned semiconductor layer.
 16. A method according to claim 1 wherein the mesa sidewalls are free of the contact layer
 17. A method of forming semiconductor device the method comprising: forming a semiconductor structure on a substrate, the semiconductor structure including a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate; forming a contact layer on the mesa surface; and forming a passivation layer on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface, the passivation layer having a via hole therein exposing a portion of the contact layer opposite the mesa surface.
 18. A method according to claim 17 wherein the contact layer substantially covers an entirety of the mesa surface.
 19. A method according to claim 17 wherein the semiconductor structure includes a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa.
 20. A method according to claim 19 wherein the semiconductor structure further includes an active layer between the P-type layer and the N-type layer.
 21. A method according to claim 19 further comprising: forming a second contact layer electrically coupled with the semiconductor structure so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer.
 22. A method according to claim 19 wherein the N-type layer is between the P-type layer and the substrate and wherein the P-type layer is between the N-type layer and the contact layer.
 23. A method according to claim 17 further comprising: forming a metal layer on the passivation layer and on the exposed portions of the contact layer opposite the semiconductor layer.
 24. A method according to claim 17 wherein the contact layer comprises a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium.
 25. A method according to claim 17 wherein the semiconductor layer comprises an epitaxial semiconductor layer.
 26. A method according to claim 17 wherein the semiconductor layer comprises a Group III-V semiconductor material.
 27. A method according to claim 26 wherein the Group III-V semiconductor material comprises a Group III-nitride semiconductor material.
 28. A method according to claim 17 wherein the mesa is configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure.
 29. A method according to claim 17 wherein the mesa sidewalls are free of the contact layer
 30. A method of forming a semiconductor device, the method comprising: forming a semiconductor structure on a substrate, the semiconductor structure including a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate; and forming a contact layer substantially covering an entirety of the mesa surface opposite the substrate.
 31. A method according to claim 30 further comprising: forming a passivation layer on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface, the passivation layer having a via hole therein so that a portion of the contact layer opposite the mesa surface is free of the passivation layer.
 32. A method according to claim 31 further comprising: forming a metal layer on the passivation layer and on the portion of the contact layer free of the passivation layer.
 33. A method according to claim 30 wherein the semiconductor structure includes a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa.
 34. A method according to claim 33 wherein the semiconductor structure further includes an active layer between the P-type layer and the N-type layer.
 35. A method according to claim 33 further comprising: forming a second contact layer electrically coupled with the semiconductor structure so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer.
 36. A method according to claim 33 wherein the N-type layer is between the P-type layer and the substrate and wherein the P-type layer is between the N-type layer and the contact layer.
 37. A method according to claim 30 wherein the contact layer comprises a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium.
 38. A method according to claim 30 wherein the semiconductor layer comprises an epitaxial semiconductor layer.
 39. A method according to claim 30 wherein the semiconductor layer comprises a+Group III-V semiconductor material.
 40. A method according to claim 39 wherein the Group III-V semiconductor material comprises a Group III-nitride semiconductor material.
 41. A method according to claim 30 wherein the mesa is configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure.
 42. A method according to claim 30 wherein the mesa sidewalls are free of the contact layer.
 43. A semiconductor device comprising: a substrate; a semiconductor structure on the substrate, the semiconductor structure including a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate; a contact layer on the mesa surface; and a passivation layer on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface, the passivation layer having a via hole therein exposing a portion of the contact layer opposite the mesa surface.
 44. A semiconductor device according to claim 43 wherein the contact layer substantially covers an entirety of the mesa surface.
 45. A semiconductor device according to claim 43 wherein the semiconductor structure includes a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa.
 46. A semiconductor device according to claim 45 wherein the semiconductor structure further includes an active layer between the P-type layer and the N-type layer.
 47. A semiconductor device according to claim 45 further comprising: a second contact layer electrically coupled with the semiconductor structure so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer.
 48. A semiconductor device according to claim 45 wherein the N-type layer is between the P-type layer and the substrate and wherein the P-type layer is between the N-type layer and the contact layer.
 49. A semiconductor device according to claim 43 further comprising: a metal layer on the passivation layer and on the exposed portions of the contact layer opposite the semiconductor layer.
 50. A semiconductor device according to claim 43 wherein the contact layer comprises a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium.
 51. A semiconductor device according to claim 43 wherein the semiconductor layer comprises an epitaxial semiconductor layer.
 52. A semiconductor device according to claim 43 wherein the semiconductor layer comprises a Group III-V semiconductor material.
 53. A semiconductor device according to claim 52 wherein the Group III-V semiconductor material comprises a Group III-nitride semiconductor material.
 54. A semiconductor device according to claim 43 wherein the mesa is configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure.
 55. A semiconductor device according to claim 43 wherein the mesa sidewalls are free of the contact layer.
 56. A semiconductor device comprising: a substrate; a semiconductor structure on the substrate, the semiconductor structure including a mesa having a mesa surface opposite the substrate and mesa sidewalls between the mesa surface and the substrate; and a contact layer substantially covering an entirety of the mesa surface opposite the substrate.
 57. A semiconductor device according to claim 56 further comprising: a passivation layer on the mesa sidewalls and on a portion of the contact layer opposite the mesa surface, the passivation layer having a via hole therein exposing a portion of the contact layer opposite the mesa surface.
 58. A semiconductor device according to claim 57 further comprising: a metal layer on the passivation layer and on the exposed portion of the contact layer opposite the semiconductor layer.
 59. A semiconductor device according to claim 56 wherein the semiconductor structure includes a P-type layer and an N-type layer wherein at least a portion of one of the P-type layer and/or the N-type layer is included in the mesa.
 60. A semiconductor device according to claim 59 wherein the semiconductor structure further includes an active layer between the P-type layer and the N-type layer.
 61. A semiconductor device according to claim 59 further comprising: a second contact layer electrically coupled with the semiconductor structure so that the first and second contact layers define an electrical path through the P-type layer and the N-type layer.
 62. A semiconductor device according to claim 59 wherein the N-type layer is between the P-type layer and the substrate and wherein the P-type layer is between the N-type layer and the contact layer.
 63. A semiconductor device according to claim 56 wherein the contact layer comprises a layer of a metal selected from aluminum, copper, gold, nickel, titanium, platinum, and/or palladium.
 64. A semiconductor device according to claim 56 wherein the semiconductor layer comprises an epitaxial semiconductor layer.
 65. A semiconductor device according to claim 56 wherein the semiconductor layer comprises a Group III-V semiconductor material.
 66. A semiconductor device according to claim 65 wherein the Group III-V semiconductor material comprises a Group III-nitride semiconductor material.
 67. A semiconductor device according to claim 56 wherein the mesa is configured to provide at least one of optical confinement or current confinement for a light emitting device in the semiconductor structure.
 68. A semiconductor device according to claim 56 wherein the mesa sidewalls are free of the contact layer. 